1. Field of the Invention
The present invention relates to an organic photoconductors for electrophotography, and more specifically, to a photoconductor for electrophotography which has an intermediate layer.
2. Description of the Prior Art
A photoconductor for electrophotography (hereinafter referred to as a photoconductor) basically comprises a photosensitive layer on a conductive substrate. Generally, however, it has an intermediate layer in order to improve adhesion between the conductive substrate and the photosensitive layer, to inhibit charge injection from the conductive substrate into the photosensitive layer, and to cover defects in the surface of the conductive substrate.
As such an intermediate layer of an inorganic type is known an Alumite layer. Japanese Patent Application Laying-open Nos. 116,160/1988, 116,161/1988 and 116,162/1988, and U.S. Pat. No. 4,800,144, for example, disclose that an Alumite layer with a thickness of several micrometers provided on an aluminum substrate is a layer having stable barrier characteristics which inhibit the injection of charges from the conductive substrate into the photosensitive layer without undergoing little influence from environmental changes.
As organic materials for the intermediate layer have hitherto been known polyvinyl alcohol, casein, and alcohol-soluble nylon.
The characteristics required of the intermediate layer include the ability to upgrade image quality by enhancing adhesion between the photosensitive layer and the conductive substrate, and improving the applicability of the photosensitive layer onto the conductive substrate by covering the surface defects of the conductive substrate. The characteristics required first of all are satisfactory electrical characteristics for a photoconductor, namely, high sensitivity and low residual potential. To achieve these desired characteristics, the above-mentioned intermediate layer of the organic type generally is made of a resin having low resistance itself. This type of intermediate layer is also required to have electrical characteristics unaffected by the environment. However, most of intermediate layers from the aforementioned organic materials are apt to be affected easily by environmental factors, especially humidity. Under low-humidity conditions, their resistance becomes high, causing fog to the resulting image. At high humidity, their resistance becomes too low, and charge potential lowers, decreasing image density. Thus, the intermediate layer using such resin is generally coated with a very small thickness of, say, 0.1 to 1 .mu.m. Such a thin film, needless to say, has a low effect of covering the defects present in the conductive substrate.
To overcome the above drawbacks, an organic type intermediate layer which functions fully even with a large thickness is under energetic development. For instance, Japanese Patent Application Publication Nos. 42,498/1987, 19,869/1988, 51,183/1989, 51,185/1989 and 60,177/1990 exemplify intermediate layers with a large thickness having a conductive fine powder dispersed in various resins. The electric conduction of these exemplified intermediate layers is attributed to electronic conduction by the conductive fine powder. Hence, even though they have a large thickness, they are assumed to show satisfactory electrical conductivity and undergo minimal influence from temperature and humidity. However, the dispersions of the conductive fine powder are subject to the precipitation or agglutination of the conductive fine powder, thus requiring careful administration of the dispersions. Furthermore, a considerable amount of the conductive fine powder has to be incorporated in the coating with the aim of imparting sufficient conductivity. As a result, the surface smoothness of the intermediate layer vanishes, and the injection of charges from the intermediate layer into the photosensitive layer is apt to occur easily. In order to give smoothness to the intermediate layer, there is further need to provide a thin layer of a resin, such as nylon or casein, onto it.
According to the above-described prior art, the intermediate layer particularly of the organic type must be able to upgrade image quality based on the improvement of adhesion between the photosensitive layer and the conductive substrate, and the improvement of the applicability of the photosensitive layer onto the conductive substrate by covering the surface defects of the conductive substrate. To fulfill this requirement, there has been proposed a method which comprises dispersing a conductive fine powder in various resins to form intermediate layers with a large thickness. However, dispersions of the conductive fine powder are subject to the precipitation or agglutination of the conductive fine powder, thus posing difficulty with the administration of the dispersions, and involving viscosity changes due to coagulation. Consequently, the surface smoothness of the intermediate layer disappears, thus arousing a new problem that a thin layer of a resin must be provided on the intermediate layer.